When a natural reservoir pressure of an oil bearing reservoir is insufficient to produce sufficient oil to make recovery economically practical or the reservoir pressure has been depleted to the uneconomic point, by previous production, it is common practice in the art to resort to techniques generically referred to as enhanced oil recovery. While such techniques have proven practical and economic in the recovery from reservoirs containing relatively light oils, the problem of oil recovery has been further complicated by the rapid decline in the availability of light oil reservoirs. As a result, operators have been forced to consider their recovery of heavier oils which, by the very nature defy, conventional methods of production and enhanced oil recovery. One technique, which has had limited success in the recovery of heavy oils, is the injection of steam into the oil bearing formation, either through a single well, in which injection of steam and production of oil are alternated (huff and puff), or injection of steam into an injection well to displace the oil from the reservoir to a production well. There are two basic methods for steam injection which are quite distinct from one another.
The most widely used steam injection technique is one in which steam is conventionally generated by a boiler located at the surface of the earth and the steam is then injected down the well to the formation to be stimulated. This technique has the obvious disadvantage that substantial losses of heat and pressure occur during transmission of the steam down the well bore. The losses are so great that at any significant reservoir depth the steam becomes hot water by the time it reaches the reservoir. In order to compensate for this loss in temperature during transmission down the well bore, insulated tubing has been proposed and, in order to compensate for loss of heat and loss of pressure, it has been suggested that the steam be generated by a miniature down hole boiler. However, this problem persists and, in addition, the steam pressure is inherently low. Thus, this technique has been denominated the "low pressure" or "low intensity" technique. Consequently, use of this technique is limited to very shallow reservoirs having very low reservoir pressures. In addition, these techniques create a more serious problem of air pollution, since flue gases from the fuels utilized to fire the boiler must be discharged to the atmosphere and such flue gases normally contain excessive amounts of nitrogen oxides and sulfur oxides. In some cases, where large concentrations of such boiler type steam generators have been utilized in a particular oil field, further use has been suspended so that air pollution standards will not be exceeded.
The second means of steam injection, referred to as the "high pressure" or "high intensity" technique, is to burn a fuel in a combustor to produce flue gas, inject water into the flue gas to produce a mixture of flue gas and steam, and inject the mixture of flue gas and steam into the reservoir. While this technique has been utilized with the generator located near the surface of the earth, thus also having the problem of heat and pressure losses due to transmission down the well, major efforts in the development of this technique has been the development of a generator which can be lowered into the well adjacent the formation of interest. In the latter instance, losses in pressure and heat during transmission down the well are eliminated. Thus, the down hole steam generator extends the depth of the reservoirs which can be treated. In addition, in order to effectively produce a mixture of flue gas and steam at a pressure sufficient for injection into a reservoir it is necessary that the combustor utilized to generate the flue gas be a "high intensity" burner, sometimes referred to as a "high velocity" or "high pressure" combustor. Accordingly, the mixture of flue gas and steam will also be at a substantially higher pressure than steam generated by a conventional boiler, thus further extending the depths and pressures at which the technique can be practiced. Therefore, this technique has been designated the "high pressure" or "high intensity" steam injection technique. In addition to the benefits gained by producing a high pressure effluent for injection, this technique has several additional distinct advantages over the low pressure technique. Most significant, it has been found that the subsurface earth formation will scrub out most of the nitrogen oxides and sulfur oxides from the flue gas and thereby essentially eliminate the problem of air pollution. Further, it has also been found that the mixture of flue gas and steam may result in increases in oil production over steam alone, apparently because of absorption of flue gases, particularly carbon dioxide, in the oil resulting in a further reduction of the oil viscosity. In a broad sense such high pressure steam generators comprise an elongated combustion chamber, means for injecting water into the flue gas and a vaporization chamber to vaporize the water and produce a mixture of flue gas and steam. Details of a highly effective high pressure steam generator of this type, which has been successfully utilized commercially, are set forth in copending U.S. application Ser. No. 354,858 filed Mar. 4, 1982 by Robert M. Schirmer. The disclosure of this application is incorporated herein by reference.
One problem in the use of high pressure steam generators is controlling the pressure within the combustion chamber and vaporization chamber so that the combustor, in particular, can be operated at its design operating conditions, since such conditions are obviously the most efficient. While the generator can be designed to operate efficiently and effectively for a particular reservoir at a particular pressure, the reservoir pressure will normally increase as effluent is injected into the reservoir and efficient use of the generator is limited to reservoir pressures which do not greatly exceed the design pressure of the generator. Such increases of reservoir pressure at which the generator can be utilized can be compensated for by increasing the duty (MM Btu/hr) of the combustor and the flow rate or velocity through the generator, primarily by increasing the air flow rate or pressure. In addition to operating less efficiently, this alternative significantly increases the fuel requirements as well as the compression requirements for compressing the air. One solution to this problem, which is set forth in the application referred to above, is to provide a fixed diameter nozzle or orifice in the downstream end of the vaporization chamber, the diameter of which has been sized for operation with choked flow (constant flow over a limited range of conditions within the generator). At a particular ratio of the pressure downstream of the nozzle throat (roughly equivalent to the reservoir pressure) to the pressure within the vaporization chamber immediately upstream of the nozzle throat, acoustic or sonic velocity of the exhaust fluids through the nozzle throat can be attained. At this critical velocity or expansion ratio, the generator, and particularly the combustor flow, will remain constant regardless of any further decreases in the back pressure (reservoir pressure). For example, in the specific generator described in the previously mentioned application, a design pressure was about 300 psi at 500.degree. F. for the exiting effluent temperature. Acoustic velocity or sonic velocity for these conditions would be about 1,662 feet/second. The critical expansion ratio through the nozzle is 0.5431 for the generator and design operating conditions. Consequently, at exit chamber pressure of 156 psig or lower the generator will operate at design conditions if it is fitted with a simple converging nozzle having throat diameter of 0.75 inches. Thus, in a typical Kern River reservoir in California where heavy oil is produced, the formation depth is 900 feet and reservoir pressure is about 35 psig. Operation of the generator at 156 psig would thus provide a pressure differential of 121 psi for pressuring the formation which should be adequate for forcing the hot combustion products and steam into the formation. However, if the pressure becomes sufficiently large such that the critical pressure ratio no longer exists, flow through the orifice will drop and will continue to do so as the downstream pressure (reservoir pressure) increases. For example, in order to maintain constant flow through the generator, the operating pressure of the generator would have to be increased. This would result in an increase in operating costs. Operation of the generator at choked flow conditions in a deep high pressure reservoir will thus be unnecessarily expensive. For a reservoir pressure of 1200 psig the generator would have to be operated at greater than 2200 psig in order to maintain a critical pressure ratio. In addition to the increased compressor costs, there would be associated design problems, such as the pressure drop across the walls of the generator, which of course would be substantially greater at higher operating pressures and thus require generator walls with greater thickness. It would, therefore, be highly desirable if the pressure at which the generator could be operated efficiently were extended and/or pressure at which the generator could be utilized could be readily adjusted to accomodate use in higher pressure reservoirs or to accomodate changes in reservoir pressure in a single reservoir.